This type of accident is known in the technical literature under the acronym CFIT for “Controlled Flight into Terrain”. Whereas, in the past, they formed a large proportion of aviation disasters, accidents of the CFIT type are nowadays mostly avoided thanks to ground avoidance maneuvers carried out by crews being prompted by warnings and alarms generated by on-board automatic systems for warning of risks of collision with the ground and with obstacles known under the acronym TAWS (Terrain Awareness & Alerting Systems), of which the GCAS (Ground Collision Avoidance System) system and the T2CAS (Terrain & Traffic Collision Avoidance System) system, developed and marketed by the Thales company, form part.
The instruction given to an aircraft crew confronted with a risk of collision with the ground or with obstacles is to undertake an avoidance manoeuvre in accordance with a pre-defined avoidance procedure which corresponds to a purely vertical avoidance manoeuvre referred to as ‘Pull-Up’, consisting in climbing using the best performance parameters of the aircraft, a manoeuvre referred to as ‘standard avoidance manoeuvre’ or, alternatively, SVRM (Standard Vertical Recovery Manoeuvre).
On-board equipment warning automatically of flight situations leading to risks of collision with the ground and with obstacles, sufficiently in advance for an appropriate avoidance manoeuvre to be effective, have been developed in recent years. Amongst these systems, the TAWS systems offer the best performance since they make use of a function referred to as FLTA (Forward-Looking Terrain Avoidance) which looks in front of the aircraft along and below its trajectory, vertically and laterally, if there is a potential risk of collision with the ground or with obstacles.
The principle of TAWS systems is based on the monitoring of the penetration of the ground and the obstacles into one or more protection volumes associated with the aircraft using a modelling of the terrain overflown. The hills and mountains of the region overflown are stored in a digital map accessible to the aircraft. The position of the aircraft with respect to the region overflown is supplied by flight equipment such as: inertial guidance system, satellite positioning receiver, baro-altimeter, radio-altimeter or a combination between several of these sensors. The protection volumes associated with the aircraft are advantageously defined in such a manner as to contain a modelling of the standard vertical avoidance manoeuvre trajectory undertaken with a variable delay starting from the trajectory followed by the aircraft predicted from the flight parameters delivered by the flight equipment of the aircraft, assuming that the aircraft maintains its ground velocity vector or its trajectory. In general, there are two protection volumes, of staged sizes, associated with the aircraft, the most forward one being used in order to give a substantial warning to the crew of the aircraft that the trajectory followed will need to be modified in the medium term in order to avoid the ground, and the nearest one being used in order to give a substantial alarm to the crew of the aircraft that they must effectively urgently undertake a vertical avoidance manoeuvre.
For more details on the concepts implemented in TAWS systems, reference may be advantageously made to the U.S. Pat. No. 5,488,563, U.S. Pat. No. 5,414,631, U.S. Pat. No. 5,638,282, U.S. Pat. No. 5,677,842, U.S. Pat. No. 6,088,654, U.S. Pat. No. 6,317,663, U.S. Pat. No. 6,480,120 and to the French Patent applications FR 2.813.963, FR 2.842.594, FR 2.848.661, FR 2.860.292, FR 2.864.270, FR 2.864.312, FR 2.867.851, FR 2.868.835.
However, an operational nuisance potentially generated by such systems is the occurrence of a spurious alert associated with an erroneous evaluation of the situation of the aircraft relative to the ground and the surrounding obstacles.
There is therefore a need in operational TAWS systems for an adaptation of the alarm triggering logic in flight situations for which the conventional methods are maladapted because of the particular local configuration of the obstacles. This could be an urban environment, for example, comprising numerous obstacles.